Sensing the surroundings of a conveyance means, in particular a motor vehicle, may be performed in principle using lidar (=light detecting and ranging), radar (=radio detecting and ranging), video, or ultrasound.
Thus, an object detection system having microwave radar sensors is known from German Published Patent Application No. 42 42 700, through which the detection of objects, even those traveling ahead of a vehicle at a great distance, is made possible. This radar sensor contributes to a vehicle safety system, in which information is continuously processed about the distance and the velocity of the vehicle in relation to the vehicles traveling ahead in a predefined angular range.
German Published Patent Application No. 44 42 189 discloses that in a system for distance measurement in the surroundings of motor vehicles, sensors having transmitting units and receiving units may be used for both transmitting and receiving information.
With the aid of range rating, according to German Published Patent Application No. 44 42 189, passive protective measures may be activated for the motor vehicle in the event of a front, side, or rear collision, for example. By exchanging the registered information, traffic situations may be judged to activate appropriate deployment systems, for example.
Furthermore, an object detection system is known from German Published Patent Application No. 196 16 038, in which an optical transmitter for a light beam having an adjustable transmission angle and an angle-resolving optical receiver are provided. The light beam emitted is modulated here in such a way that the position of the object within the angular range of the light beam emitted may also be determined up to a specific distance from the phase difference of the transmitted light beam and the received light beam.
A sensor system for automatically determining the relative position between two objects is disclosed in German Published Patent Application No. 196 22 777. This conventional sensor system includes a combination of an angle-independent sensor and an angle-dependent sensor. The sensor which does not resolve the angle and is therefore angle-independent is implemented as a sensor which analyzes the distance to an object via a run-time measurement. Radar, lidar, or ultrasonic sensors are suggested as possible sensors.
The angle-dependent sensor includes a geometrical system of optical-electronic transmitters and receivers, which are positioned in the form of light barriers. The sensors, which both cover a joint detection region, are positioned closely next to one another. In order to determine a relative position to the object, the distance to the object is determined using the angle-independent sensor and the angle to the object is determined using the angle-resolving sensor.
The relative position to the object is known on the basis of the distance and angle to the object. As an alternative to the cited system of optical-electronic transmitters and receivers, a use of two sensors is suggested which jointly determine the angle to the object according to the triangulation principle.
A method and a device for object detection having at least two distance-resolving sensors attached to a motor vehicle, whose detection ranges at least partially overlap, is known from German Published Patent Application No. 199 49 409.
For this purpose, according to German Published Patent Application No. 199 49 409, means are provided in order to determine relative positions of possible detected objects in relation to the sensors in the overlap region according to the triangulation principle; possible illusory objects, which result from the object determination, may be determined through dynamic object observations.
Finally, an object detection system, in particular for a motor vehicle, is suggested in German Published Patent Application No. 100 11 263, the object detection system having multiple object detectors and/or modes of operation, using which different detection ranges and/or detection regions are registered.
For this purpose, according to German Published Patent Application No. 100 11 263, an object detector may be a radar sensor, which has a relatively large detection range with a relatively small angular range in a first mode of operation, and has a relatively small detection range in relation thereto, with an enlarged angular range, in a second mode of operation.
In addition, it is generally known per se that a distance measurement may be performed using a pulse radar, in which a carrier pulse having a square wave envelope of an electromagnetic oscillation is emitted in the gigahertz range.
This carrier pulse is reflected on the target object, and the target distance may be determined from the time span between the emission of the pulse and the incidence of the reflected radiation, and the relative speed of the target object may also be determined with restrictions by making use of the Doppler effect.
A pulse radar device of this type is known from German Published Patent Application No. 199 26 787. In this case, a transmit switch is switched on and off by the pulses of the generator, so that during the pulse duration a high-frequency wave generated by an oscillator and conducted via a fork to the transmission switch is switched through to the transmit antenna.
According to German Published Patent Application No. 199 26 787, a receive part also receives the output signal of the generator. The receive signal, i.e., a radar pulse reflected on an object, is mixed during a predefined time gate with the oscillator signal, which reaches a mixer via a receive switch, and analyzed.
U.S. Pat. No. 6,067,040 also describes a transmit switch, which is switched on and off by pulses of the generator. Separate channels are provided for in-phase/quadrature (I/Q) signals for receiving the reflected radar pulses.
As in the pulse radar device according to German Published Patent Application No. 199 26 787, the received signal is also mixed and analyzed only during a predefined time gate according to U.S. Pat. No. 6,067,040, for which a receive-side pulse modulator or pulse switch is located upstream from a power divider provided for dividing the local oscillator (LO) signals to the mixer in the receive-side I/Q (in-phase quadrature) branches.
This has the disadvantage, however, that no multi-receiver system may be implemented and no simultaneous analysis of multiple different receive cells is possible.
In contrast, in the suggestion according to German Published Patent Application No. 101 42 170, two separately controllable receive-side pulse modulators are provided; the continuous signal of the high-frequency source, which also controls the transmit-side pulse modulator, is switchable to one receive-side mixer via each of these modulators. This means that, in contrast to the suggestion according to U.S. Pat. No. 6,067,040, each mixer in a receive branch has the signal of the high-frequency source applied to it at different points in time and also may be connected to the signal of the high-frequency source for different lengths of time. In this way, different modes of operation are made possible, which are reversible in a relatively simple way.
A measuring principle based on the emission of pulses is also described, for example, in the textbook by Albrecht Ludloff, “Handbuch Radar und Radarsignalverarbeitung” (“Handbook of Radar and Radar Signal Processing”), pp. 2–21 to 2–44, Vieweg-Verlag, 1993. The textbook by Merrill Ivan Skolnik, “Introduction to Radar Systems,” pp. 74 et seq., McGraw-Hill Publishing Company, may also be noted as further literature.
Typically, multiple radar sensors are necessary for the individual conflict situations in the surroundings of a motor vehicle to reliably activate the above-mentioned occupant protection systems in the motor vehicle; for example, a pre-crash recognition is necessary in order to allow prior registration of an object which represents a danger to the vehicle occupants in the event of a collision. In this way, it is to be possible to activate protective systems such as an airbag, belt tightener, or side airbag in a timely manner, in order to thus achieve the greatest possible protective effect.
The registration and/or monitoring of the traffic situation, in particular in proximity to the motor vehicle, may additionally be useful for multiple further applications. These include parking aids, aids for monitoring the “blind spot,” and assistance in “stop and go” traffic, in which the distance to the vehicle traveling ahead is determined in order to be able to stop and go automatically.
For this purpose, multiple radar sensors, each having different requirements tailored to the measuring task, are typically used, the requirements essentially differing in the range and the analysis time, because each of these functions has specific registration ranges and different measuring cycle times; in principle, universal sensors may be operated jointly via a specially tailored bus system and connected together using an analysis unit; however, for reasons of performance, all distance ranges within a proximate range often may not be processed optimally in an analysis time which is relatively short for reliable functioning.
For these reasons, a device and a method for registering and analyzing objects in the surroundings of a motor vehicle is suggested in German Published Patent Application No. 199 63 005.
For this purpose, according to German Published Patent Application No. 199 63 005, the surroundings of the motor vehicle are registered by making use of a transmit signal of one pulse radar sensor at a time in one or more receive branches in such a way that different distance ranges are analyzed in parallel and/or sequentially; however, neither the device nor the method according to German Published Patent Application No. 199 63 005 is capable of also providing corresponding angle information in regard to the object to be detected.
Therefore, if radar sensors are used for registering the surroundings of the motor vehicle, the distance and the velocity of objects, such as other vehicles, are to be detected in the detection range of the radar sensor system. At close range, 24 gigahertz pulse radar sensors are used, for example, using which a Doppler analysis may be performed through a coherent receiver principle; this allows the velocity to be determined by analyzing the phase change over time (=“Doppler effect”).
Various more conventional methods for measuring velocities are known:
In range rating, the radial distance between the sensor and the object is measured in discrete time intervals of the scan cycle, typically ten milliseconds. The velocity to be measured results from the differential quotients of distance change and time interval.
However, range rating, i.e., position comparison from multiple scans in the interval of the cycle time, results in diverse problems:                At higher velocities of the object, restrictions to the unambiguity of the peak assignment may occur from scan to scan, because the distance change exceeds the width of the detection gate. In case of multiple moving objects in the detection space, this results in the radar reflections detected from the corresponding objects only being able to be assigned with difficulty.        Real objects exhibit strong fluctuation in their reflectivity. This is true in particular for quasi-optical reflection behavior in the microwave range, because the smallest changes in the angle of incidence of the reflection plane to the wavefront cause a strong change in the reflection cross section (RCS). The resulting change in the signal amplitude is distributed stochastically and may result in detection failures from scan to scan. The differential filter used must therefore process a high number of scans and thus has an undesirably high latency time.        The measurement of the velocity is imprecise because sequential measurements on the target object may find reflection centers having different radial distances; if multiple reflections on the object are superimposed into an overall reflection within the measuring gate, the phase offset of the individual reflections may change strongly from scan to scan, so that the focal point of the superimposed reflections shifts strongly. This results in measurement fuzziness.        
In closing velocity (CV) measurement, the detection gate (range gate) is fixed at a defined radial distance having an envelope width of approximately twenty centimeters. An object traveling through this detection gate causes n phase angle rotations (n=16=20 centimeters divided by the wavelength λ). The phase change over time is the angular velocity and is proportional to the velocity of the object.
However, CV measurement also exhibits diverse problems:                During the CV measurement, the detection gate, whose width is approximately twenty centimeters and which is activated by the scan, is statically fixed at one position;        
during this measurement, the remaining detection region is not scanned. Therefore, other objects which may be functionally relevant are ignored.                As a result, it must be decided very reliably, through prior range rating in the normal scan operation, that a target is on a collision course in order to trigger the event transition into CV mode.        If it is a false triggering, the system must fall back into normal scan operation rapidly. Through this method, significant periods of time may arise in which there is no target detection.        If multiple objects having different velocities travel through the detection gate simultaneously, this generates a superposition of multiple Doppler frequencies, due to which simple frequency counting methods fail.        
Because the convolution of the received signal using a detection gate is suspended during continuous wave (CW) measurement and instead of this there is continuous receiving operation, the Doppler frequencies of all moving objects in the range of the measuring sensitivity of the sensor system are measured simultaneously and without uniquely locating them.
However, CW measurement also exhibits diverse problems:                The measurement of the velocity is not location-specific and may therefore not be uniquely assigned to a specific object.        The Doppler frequencies of multiple objects are superimposed, so that simple frequency counting methods fail; instead of this, precise multifrequency-capable fast Fourier transforms (FFT) are used.        The measuring range is not delimited in a defined way, but rather is determined from antenna pattern, receiver sensitivity, and target size (RCS).        